The present embodiments relate to a high-voltage generator for an X-ray device.
A high-voltage generator used to supply an X-ray device has in the customary configuration an inverter on the input side, a rectifier on the output side and a transformer connected between the inverter and the rectifier. The inverter in this arrangement feeds a primary winding of the transformer with an inverter voltage that the transformer transforms into a rectifier voltage fed to the rectifier. The rectifier voltage is then converted by the rectifier into an output voltage to be fed to the X-ray device. The rectifier used in such a high-voltage generator is customarily a passive rectifier, (e.g., a rectifier that is equipped solely with diodes).
High-voltage generators for X-ray devices are usually be designed for both high output voltages of up to 150 kV and high peak power levels of up to 100 kW. The transformers used to generate the high voltage are customarily operated at a frequency of approximately 50 kHz in order to permit a compact design despite the high power level and voltage.
The average heat loss to the environment over time of an X-ray device is generally low relative to the aforementioned peak power, however, because X-ray devices are usually operated either in a pulsed manner or at low power in continuous duty. The associated high-voltage generators, in particular the transformers thereof, therefore do not generally have to be designed for continuous duty at peak power.
This makes it possible to realize the transformer in a very compact form. Such a transformer often has relatively high stray inductance. This is disadvantageous, as under load the stray inductance causes a voltage drop that has to be compensated for with appropriate measures.
It is possible in principle to compensate for this voltage drop by adopting an overdimensioned transformation ratio n=N2/N1 for the transformer (i.e., the ratio of the number of windings N2 of the secondary winding to the number of windings N1 of the primary winding). The transformation ratio is selected in this case to be higher than the voltage transformation ratio to be achieved so that the required output voltage is reached even when the high-voltage generator is operated at peak power. An increase in the transformer transformation ratio, however, has the unfavorable effect of also increasing the rms value of the inverter currents. The semiconductor components of the inverter have to be designed to accommodate these elevated inverter currents, which leads to increased production expense.
It is therefore customary for cost reasons to insert a capacitor in series with the primary winding to compensate for the stray inductance of the transformer. The resulting circuit is also referred to as a “series resonant converter” because of the fact that the capacitor forms a series resonant circuit with the primary winding. It is possible to realize a virtually load-independent output voltage with the series resonant converter if the resonant frequency of the series resonant circuit is tuned to the switching frequency of the inverter. The transformation ratio of the transformer consequently need not be overdimensioned in relation to the maximum voltage transformation ratio to be achieved with this solution. The output voltage may be regulated by varying the switching frequency and/or the duty cycle of the inverter. The behavior of the series resonant converter is unfavorable from a regulation technology perspective though because the serial oscillator circuit forms a second order system at the resonant frequency. Moreover with the series resonant converter, “hard switching” (i.e., the switching on of the semiconductor switches of the inverter under voltage) may only be avoided by appropriate variation of the switching frequency. This complicates regulation and disadvantageously makes it necessary to measure the current zero-crossing point of the inverter current.
US 2004/0218404 A1 (specifically FIG. 11 therein) discloses a step-up DC-DC converter that has a three-legged transformer core (E core). Each of the two outer legs of the E core in this arrangement is wound with a primary winding that is activated cyclically by a transistor. Each of the two outer legs of the E core also has a secondary winding where these secondary windings are connected in series with a passive bridge rectifier. The central leg of the E core, which has an air gap, bears a third secondary winding connected via a diode in parallel with the bridge rectifier.
Other DC-DC converters, some of which are envisaged as a high-voltage generator for an X-ray device, are disclosed in US 2008/0247195 A1, US 2008/0130323 A1 and US 2004/0037092 A1.